Regulation or cyclic GMP (cGMP) levels by phosphodiesterase (PDE) is central to the visual transduction pathway in retinal photoreceptor cells. While the initial events leading to PDE activation have been extensively studied, we know much less about the pathways that regulate PDE activity during the recovery of the dark-adapted state or during light or bleaching adaptation. Furthermore, it is known that mutations in the catalytic and regulatory domains of the PDE catalytic subunits can result in retinitis pigmentosa and congenital stationary night-blindness in humans. Furthermore, disruptions of the inhibitory gamma subunit of PDE lead to retinal degeneration in animal models. The overall objective of the research is to define the regulatory mechanisms residing within the PDE holoenzyme, as well as to understand how other PDE-interacting proteins modulate the extent and lifetime of PDE activation during visual transduction. The specific aims of this p proposal are: (1) To define the distinct conformational states of PDE during its activation and inactivation, and to elucidate the role of the regulatory GAF domains and the inhibitory gamma subunit in these allosteric transitions. (2) To identify and characterize known (e.g., delta, GARP2) and novel proteins that form a signaling complex with PDE, and to determine their regulatory significance for photoreceptor recovery and adaptation. Because precise regulation of PDE activity is critical to signal transduction and metabolism in photoreceptors, any defect in PDE structure, function, or regulation can have serious consequences, including visual impairment, retinal degeneration, and blindness. Increased understanding of the regulation of PDE will aid in designing effective therapies to intervene in certain forms of retinal degeneration.